1. Field of the Invention
The present invention relates to a counter electrode having a porous carbon material layer for a dye-sensitized photovoltaic cell and a method of preparing the same. More particularly, the present invention relates to a counter electrode for a dye-sensitized photovoltaic cell, which has superior durability and high energy-conversion efficiency, and to a method of preparing the counter electrode.
2. Description of Related Art
Generally, a dye-sensitized photovoltaic cell includes a photo-electrode, a counter electrode, and electrolyte. The photo-electrode is generally formed of a transparent conductive oxide substrate in which metal oxide nanoparticles having a wide band gap energy and photosensitive dye are absorbed. The counter electrode is formed of a transparent conductive oxide substrate coated with platinum (Pt).
When the dye-sensitized photovoltaic cell is exposed to sun light, the photosensitive dye absorbing the sun rays is in an excited state and thus transmits electrons to a conduction band of the metal oxide. The conducted electrons move to the electrode and flow to an external circuit to transmit electrical energy to the external circuit. The electrons are subsequently reduced in energy while transmitting the electrical energy to the external circuit and move to the counter electrode, after which the photosensitive dye is supplied with electrons from the counter electrode through the electrolyte, thereby returning to its initial state. At this point, the electrolyte functions to receive the electrons from the counter electrode by an oxidation-reduction reaction and transfer the electrons to the photosensitive dye.
In most cases, the counter electrode is coated with platinum (Pt). Platinum has the advantage of having a high degree of electrical conductivity, and also has a catalytic property (oxidation-reduction reaction). However, platinum is expensive and limited in increasing a surface area where catalysis occurs. Therefore, limitations are encountered with respect to increasing a catalysis speed of the dye-sensitized photovoltaic cell.
Furthermore, when a module of the photovoltaic cell is large in size, expensive equipment such as a large-sized sputtering facility is used to prepare the platinum counter electrode or the counter electrode is prepared through a wet coating method that uses a large amount of platinum compound. This is economically infeasible as manufacturing costs are increased.
In order to solve the above-described drawbacks, studies are being conducted for finding suitable components for the counter electrode that can substitute for platinum.
For example, it has been reported that a carbon material can provide energy-conversion efficiency that is almost identical to that of platinum (M. Gratzel et al., Solar Energy Materials and Solar Cells 44 (1996) pp. 99-117) (K. Imoto et al., Solar Energy Materials and Solar Cells 79 (2003) pp. 459-469). In addition, a photovoltaic cell having a counter electrode coated with a carbon material or formed of a mixture slurry of a carbon material and a binder has been proposed.
However, since the conventional carbon counter electrode is not durable, the performance of the cell is sharply deteriorated when it is used for long periods of time. More particularly, under a situation where the carbon counter electrode is applied with a heat cycle load, the counter electrode may be delaminated or cracked, or carbon powders may float.
In order to enhance the durability of the counter electrode, a method of preparing the counter electrode using carbon powders mixed with binders such as metal oxide particles or metal particles that can be finely dispersed has been proposed (Japanese laid-open patent No. 2005-302390).
However, at this point, there is still a need for a counter electrode that has sufficient durability that is required for use over long periods of time.